There are a number of radio frequency identification techniques known. The present invention relates to a transponder system which receives an interrogation radio wave, and passively emits radio waves derived from the interrogation radio wave, modulated with information from the transponder. These systems typically operate according to one of two principles: a modulated backscatter based on a sequential alteration in antenna impedance; and an "echo" pattern of the interrogation beam consisting of a characteristic transfer function having a set of internal delays.
Many known systems employ the 915 MHz band (about 905-925 MHz), which was made available by regulations in much of the world, including the U.S. Presently, however, users are being encouraged to migrate to different bands. Further, in sensing an echo pattern of a passive acoustic transponder device, typically the interrogation radio wave is made non-stationary, for example in a chirp waveform. These chirp techniques are also being discouraged by regulation. The first type of RF-ID transponder tag includes an electronic circuit, e.g., CMOS, to store digital ID data which is then modulated onto a received signal by means of an RF circuit, e.g., a GaAs MESFET, transistor or controlled diode. Power for the data storage and modulating circuit may be derived from an interrogating RF beam or another power source, such as a battery, and the backscatter emission is also derived from the beam. In this type of system, the interrogating RF beam is generally of fixed frequency or direct sequence spread spectrum (See, U.S. Pat. No. 4,888,591, expressly incorporated herein by reference), with the resulting modulated signal at the same carrier frequency, with AM, FM, PSK, QAM or another known modulation scheme employed. In order to provide separation between the received and transmitted signals, the modulated output may be, for example, transmitted as a harmonic of the interrogating RF beam. Such a system is disclosed in U.S. Pat. No. 4,739,328, expressly incorporated herein by reference.
A second type of RF-ID tag includes a passive acoustic wave device, in which an identification code is provided as a characteristic time-domain reflection pattern in a retransmitted signal, in a system which generally requires that the signal emitted from an exciting antenna be non-stationary with respect to a signal received from the tag. This ensures that the reflected signal pattern is distinguished from the emitted signal. In such a device, received RF energy, possibly with harmonic conversion, is emitted onto a piezoelectric substrate as an acoustic wave with a first interdigital electrode system, from whence it travels through the substrate, interacting with reflector elements in the path of the wave, and a portion of the acoustic wave is ultimately received by the interdigital electrode system and retransmitted. These devices do not require a semiconductor memory. The propagation velocity of an acoustic wave in a surface acoustic wave device is slow as compared to the free space propagation velocity of a radio wave. Thus, assuming that the time for transmission between the radio frequency interrogation system is short as compared to the acoustic delay, the interrogation frequency should change such that a return signal having a minimum delay may be distinguished, and the interrogation frequency should not return to that frequency for a period longer than the maximum acoustic delay period. Generally, such systems are interrogated with a pulse transmitter or chirp frequency system.
Because the encoded information normally includes an identification code which is unique or quasi-unique to each transponder, and because the transponders of this type are relatively light weight and small and may be easily attached to other objects to be identified, the transponders are sometimes referred to as "labels" or "tags". The entire system, including the interrogator/receiver apparatus and one or more transponders, which may be active or passive, is therefore often referred to as a "passive interrogator label system" or "PILS".
In its simplest form, the acoustic wave RF-ID transponder systems include a radio frequency transmitter capable of transmitting RF pulses of electromagnetic energy. These pulses are received at the antenna of a passive transponder and applied to a piezoelectric "launch" transducer adapted to convert the electrical energy received from the antenna into acoustic wave energy in the piezoelectric material. Upon receipt of a pulse, an acoustic wave is generated within the piezoelectric material and transmitted along a defined acoustic path. This acoustic wave may be modified along its path, such as by reflection, attenuation, variable delay, and interaction with other transducers.
When an acoustic wave pulse is reconverted into an electrical signal it is supplied to an antenna on the transponder, which may be the same antenna which receives the interrogation wave or a different antenna, and transmitted as RF electromagnetic energy. This energy is received at a receiver, preferably at or near the same location as the interrogating transmitter, and the information contained in this response to an interrogation is decoded.
In systems of this general type, the information code associated with and which identifies the passive transponder is built into the transponder at the time that a layer of metallization is fully defined on the substrate of piezoelectric material. This metallization also defines the antenna coupling, launch transducers, acoustic pathways and information code elements, e.g., reflectors and phase delays. Thus, the information code in this case is non-volatile and permanent. The information is present in the return signal as a set of characteristic perturbations of the interrogation signal, such as delay and specific attenuation pattern. In the case of a tag having launch transducers and a variable pattern of reflective elements, the number of possible codes is N.times.2.sup.M where N is the number of acoustic waves (paths) launched by the transducers and M is the number of variable (reflective and/or delay) element positions for each transducer. Thus, with four launch transducers each emitting two acoustic waves, and a potential set of eight variable reflective elements in each acoustic path, the number of differently coded transducers is 4.sup.8 =2048. Therefore, for a large number of potential codes, it is necessary to provide a large number of launch transducers and/or a large number of reflective elements. However, efficiency is lost with increasing complexity, and a large number of distinct acoustic waves reduces the signal strength of the signal encoding the information in each.
The passive acoustic transponder tag thus includes a multiplicity of "signal conditioning elements", i.e., delay elements, reflectors, and/or amplitude modulators, coupled to receive the first signal from a transponder antenna. Each signal conditioning element provides an intermediate signal having a known delay and a known amplitude modification to the first signal. Typically, the signals representing each of multiple acoustic paths are recombined for transmission through a single antenna in a signal combining element, to produce the second signal. As described above, the signal modification elements and/or the signal combining element impart a known, unique informational code to the second signal. A receiver, generally positioned proximate to the interrogator, received the reply signal and processes the information to determine the coding of the passive acoustic transponder. Because the frequency of the interrogation signal changes over time, the received response of the tag, delayed and/or reflected due to the internal structures, is generally at a different frequency than the simultaneously emitted signal, thus distinguishing the interrogation signal from the reply signal.
The passive acoustic transponder receiving and decoding apparatus in a known system receives the reply signal from the transponder and mixes the reply signal with a representation of the interrogation signal in a four quadrant mixer, producing as an output a signal containing the difference frequencies (or frequencies derived from the difference frequencies) of the interrogation and reply signals, respectively. A signal processor detects the phases and amplitudes of the respective difference frequencies to determine the informational code associated with the interrogated transponder. Where the code is provided as a set of time delays, the signal processor performs a time-to-frequency transform (Fourier transform) on the received signal, to assist in determination of the various delay parameters. The nominal, known delay times provided in the transponder consist of a common, nominal, known delay T.sub.0 for a group of the signal delay means (reflectors), plus nominal, known differences in delay time (.DELTA.T.sub.1, .DELTA.T.sub.2 . . . .DELTA.T.sub.i) between intermediate signals produced by chronologically successive ones of the signal delay elements in the group. In order to calculate the time delays from received reflection or echo patterns, a frequency analysis or time-frequency transform, e.g., Fourier transform, is performed, which converts the set of time mapped data into frequency/phase mapped data. The characteristic delays of the transducer then appear in the transformed data set at the receiver as signal energy having a time delay. Alternately, a set of matched filters may be implemented, and the outputs analyzed.
Systems for interrogating a passive transponder employing acoustic wave devices, carrying amplitude and/or phase-encoded information are disclosed in, for example, U.S. Pat. Nos. 4,059,831; 4,484,160; 4,604,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208; 4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790; 4,951,057; 5,095,240; and 5,182,570, expressly incorporated herein by reference. Other passive interrogator label systems are disclosed in the U.S. Pat. Nos. 3,273,146; 3,706,094; 3,755,803; and 4,058,217. The following references are hereby expressly incorporated by reference for their disclosure of RF modulation techniques, transponder systems, information encoding schemes, transponder antenna and transceiver systems, excitation/interrogation systems, and applications of such systems: U.S. Pat. Nos. 2,193,102; 2,602,160; 2,774,060; 2,943,189; 2,986,631; 3,025,516; 3,090,042; 3,206,746; 3,270,338; 3,283,260; 3,379,992; 3,412,334; 3,480,951; 3,480,952; 3,500,399; 3,518,415; 3,566,315; 3,602,881; 3,631,484; 3,632,876; 3,699,479; 3,713,148; 3,718,899; 3,728,632; 3,754,250; 3,798,641; 3,798,642; 3,801,911; 3,839,717; 3,859,624; 3,878,528; 3,887,925; 3,914,762; 3,927,389; 3,938,146; 3,944,928; 3,964,024; 3,980,960; 3,984,835; 4,001,834; 4,019,181; 4,038,653; 4,042,906; 4,067,016; 4,068,211; 4,068,232; 4,069,472; 4,075,632; 4,086,504; 4,114,151; 4,123,754; 4,135,191; 4,169,264; 4,197,502; 4,207,518; 4,209,785; 4,218,680; 4,242,661; 4,287,596; 4,298,878; 4,303,904; 4,313,118; 4,322,686; 4,328,495; 4,333,078; 4,338,587; 4,345,253; 4,358,765; 4,360,810; 4,364,043; 4,370,653; 4,370,653; 4,388,524; 4,390,880; 4,471,216; 4,472,717; 4,473,851; 4,498,085; 4,546,241; 4,549,075; 4,550,444; 4,551,725; 4,555,618; 4,573,056; 4,599,736; 4,604,622; 4,605,012; 4,617,677; 4,627,075; 4,641,374; 4,647,849; 4,654,512; 4,658,263; 4,739,328; 4,740,792; 4,759,063; 4,782,345; 4,786,907; 4,791,283; 4,795,898; 4,798,322; 4,799,059; 4,816,839; 4,835,377; 4,849,615; 4,853,705; 4,864,158; 4,870,419; 4,870,604; 4,877,501; 4,888,591; 4,912,471; 4,926,480; 4,937,581; 4,951,049; 4,955,038; 4,999,636; 5,030,807; 5,055,659; 5,086,389; 5,109,152; 5,131,039; 5,144,553; 5,163,098; 5,193,114; 5,193,210; 5,310,999; 5,479,160; and 5,485,520. In addition, foreign patents CH346388; DE1295424; DE2926836; DE969289; EP0207020; FR2260115; GB1130050; GB1168509; GB1187130; GB2103408; GB2247096; GB774797; GB987868; JP0138447; JP0189467; JP116054; JP5927278; and NE1566716. The following references are also of interest: "IBM Technical Disclosure Bulletin", (vol. 20, No. 7; 12/77), pp. 2525-2526.; "IEEE Transactions on Vehicular Technology", (vol. VT-26, No. 1), 2/77; p. 35.; A. R. Koelle et al. "Short-Range Radio-Telemetry for Electronic Identification using Modulated RF Backscatter", by A. (Proc. of IEEE, 8/75; pp. 1260-1261).; Baldwin et al., "Electronic Animal . . . Monitoring", 1973.; Electronic Letters, December 1975, vol. 11, pp. 642-643.; Encyclopedia of Science and Technology; vol. 8, pp. 644-647 (1982).; Federal Information Processing Standards Publication 4A, Jan. 15, 1977, Specifications for the Data Encryption Standard.; IEEE Transactions, Henoch et al., vol. MTTT-19, No. 1, January 1971.; IEEE Transactions, Jaffe et al., pp. 371-378, May 1965.; IRE Transactions, Harrington, pp. 165-174, May 1962.; IRE Transactions, Rutz, pp. 158-161, March 1961.; J. Lenk, Handbook of Microprocessors, Microcomputers and Minicomputers; p. 51 (1979).; Koelle et al., "Electronic Identification . . . Monitoring", 7/73 to 6/74, pp. 1-5.; P. Lorrain et al., EM Fields and Waves; Appendix A, (1970).; Proceedings of IRE, March 1961, pp. 634-635; RCA Review, vol. 34, 12/73, Klensch et al., pp. 566-579.; RCA Review, Sterzer, 6/74, vol. 35, pp. 167-175.; Reports on Research, September-October 1977, vol. 5, No. 2.
Signal mixers are well known structures. These devices typically employ a non-linear element which intermodulates concurrent signals. This non-linear element may be as simple as a diode, or a more complex device.